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Jean Philippe Duvel

Abstract

Using METEOSAT data in the ISCCP B2 format, we study the mean radiation fields and their fluctuations during Northern Hemisphere summer (June, July, August) of 1983, 1984 and 1985, for regions of 5°×5° located from 50°N to 50°S and from 60°E to 60°W. The study is performed for the IR atmospheric window channel (10.5–12.5 μm) and the water vapor band (5.7–7.1 μm). The year-to-year differences of the mean fields delineate large regions of positive and negative anomalies with principally a zonal distribution. This suggests that interannual perturbations in the large-scale meridional circulation have a strong influence on the radiation field, principally by way of convective activity.

To study diurnal variations, we separate coherent diurnal variance obtained by compositing over the 3 months, and the total intradiurnal variance obtained by integration of the power spectra over periods lower than 1 day. In the IR window, the coherent diurnal variance, expressed as a percentage of the total intradiurnal variance, is stronger over subsidence areas, reaching values greater than 98% over desert regions due to surface temperature and values near 70% over ocean regions due to diurnal variations of stratiform cloudiness. Over ITCZ or midlatitude regions, this percentage is lower. In the WV band, the latitudinal distribution presents maximum values of this percentage between the equator and 10°N (>35%) with a progressive decrease up to 30° of latitude (<5%) in both hemispheres. The coherent diurnal variation is larger (up to 50%) over central and eastern Africa and related to convective activity over highlands.

Spectral analysis of interdiurnal fluctuations reveals a progressive shift of the dominant time scales from short time scales (1–2.5 day band) over convective zones to periods longer than 9.2 days over subsidence areas. Regional aspects are revealed by mapping the spectral variance in selected frequency bands as a percentage of the total interdiurnal variance. Over subsidence areas and many other regions where the IR signal depends strongly on surface temperature and lower atmospheric levels inaccessible to the WV channel, the strong coherence between the two channels suggests that the same time scales dominate over the entire vertical extent from low to middle troposphere.

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Hanh Nguyen and Jean-Philippe Duvel

Abstract

Spectral analysis of the outgoing longwave radiation (OLR) time series over equatorial Africa reveals large oscillations of the convection with periods of between 3 and 6 days. In March and April, when the intertropical convergence zone (ITCZ) migrates northward and crosses equatorial Africa, this periodic behavior is most pronounced with a marked peak at 5–6 days. Robust horizontal and vertical patterns, consistent with a convectively coupled Kelvin wave, can be extracted by a simple composite technique based only on the phase of the convective oscillations over equatorial Africa. The composite reveals differences between continental and adjacent oceanic regions. Over the continent, the stronger oscillation of the convection is associated with larger temperature and moisture anomalies near the surface, suggesting an influence of diabatic processes on the amplitude of the perturbations. Some convective events over equatorial Africa are triggered by waves propagating eastward over the equatorial Atlantic. However, this cannot explain the robust periodic behavior observed over equatorial Africa because the convective variability over the Amazon basin and the equatorial Atlantic have different spectral characteristics with no marked peak at 5–6 days in March and April.

The mesoscale convective systems embedded in these synoptic disturbances are studied using satellite brightness temperature at higher spatial (0.5°) and temporal (3 h) resolution than the OLR (respectively, 2.5° and daily average). The diurnal and the wave modulations of occurrence, size, and life cycle of the mesoscale convective systems are inspected. These systems are generated preferentially over the western slopes of the Rift Valley highlands. They propagate west-southwestward over the Congo basin where they reach their maximum size. The 5–6-day perturbations do not modify the diurnal triggering of convective systems notably, but the perturbations do modify their development into larger organized convection, especially over the Congo basin. The implication of these results for understanding the physical source of these 5–6-day perturbations is discussed.

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Hugo Bellenger and Jean-Philippe Duvel

Abstract

During periods of light surface wind, a warm stable layer forms at the ocean surface with a maximum sea surface temperature (SST) in the early afternoon. The diurnal SST amplitude (DSA) associated with these diurnal warm layers (DWLs) can reach several degrees and impact the tropical climate variability. This paper first presents an approach to building a daily time series of the DSA over the tropics between 1979 and 2002. The DSA is computed over 2.5° of latitude–longitude regions using a simple DWL model forced by hourly-interpolated surface radiative and turbulent fluxes given by the 40-yr ECMWF Re-Analysis (ERA-40). One advantage of this approach is the homogeneity of the results given by the relative homogeneity of ERA-40. The approach is validated at the global scale using empirical DWL models reported in the literature and the Surface Velocity Program (SVP) drifters of the Marine Environmental Data Service (MEDS). For the SVP dataset, a new technique is introduced to derive the diurnal variation of the temperature from raw measurements.

This DWL time series is used to analyze the potential role of DWLs in the variability of the tropical climate. The perturbation of the surface fluxes by DWLs can give a cooling of the ocean mixed layer as large as 2.5 K yr−1 in some tropical regions. On a daily basis, this flux perturbation is often above 10 W m−2 and sometimes exceeds 50 W m−2. DWLs can be organized on regions up to a few thousand kilometers and can persist for more than 5 days. It is shown that strong DWLs develop above the equatorial Indian Ocean during the suppressed phase of the intraseasonal oscillation (ISO). DWLs may trigger large-scale convective events and favor the eastward propagation of the ISO convective perturbation during boreal winter. This study also suggests that the simple approach presented here may be used as a DWL parameterization for atmospheric general circulation models.

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Jean Philippe Duvel and Jérôme Vialard

Abstract

Since the ISV of the convection is an intermittent phenomenon, the local mode analysis (LMA) technique is used to detect only the ensemble of intraseasonal events that are well organized at large scale. The LMA technique is further developed in this paper in order to perform multivariate analysis given patterns of SST and surface wind perturbations associated specifically with these intraseasonal events. During boreal winter, the basin-scale eastward propagation of the convective perturbation is present only over the Indian Ocean Basin. The intraseasonal SST response to convective perturbations is large and recurrent over thin mixed layer regions located north of Australia and in the Indian Ocean between 5° and 10°S. By contrast, there is little SST response in the western Pacific basin and no clear eastward propagation of the convective perturbation. During boreal summer, the SST response is large over regions with thin mixed layers located north of the Bay of Bengal, in the Arabian Sea, and in the China Sea. The northeastward propagation of the convective perturbation over the Bay of Bengal is associated with a standing oscillation of the SST and the surface wind between the equator and the northern part of the bay. In fact, many intraseasonal events mostly concern a single basin, suggesting that the interbasin organization is not a necessary condition for the existence of coupled intraseasonal perturbations of the convection.

The perturbation of the surface wind tends to be larger to the west of the large-scale convective perturbation (like for a Gill-type dynamical response). For eastward propagating perturbations, the cooling due to the reinforcement of the wind (i.e., surface turbulent heat flux) thus generally lags the radiative cooling due to the reduction of the surface solar flux by the convective cloudiness. This large-scale Gill-type response of the surface wind also cools the surface to the west of the basin (northwest Arabian Sea and northwest Pacific Ocean), even if the convection is locally weak. An intriguing result is a frequently occurring small delay between the maximum surface wind and the minimum SST. Different explanations are invoked, like a rapid surface cooling due to the vanishing of an ocean warm layer (diurnal surface warming due to solar radiation in low wind conditions) as soon as the wind increases.

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Carsten Standfuss, Michel Viollier, Robert S. Kandel, and Jean Philippe Duvel

Abstract

A regional (2.5° × 2.5° resolved) diurnal (hourly) albedo climatology for low and midlatitudes is derived for each month from the 5⅓-yr narrow-field-of-view data record obtained from the Earth Radiation Budget Satellite (ERBS). It is used in a quasi-operational diurnal interpolation/extrapolation procedure (DIEP) to calculate regional monthly means of the reflected shortwave radiation flux (RSR) from instantaneous albedo observations. This climatological approach (CDIEP) replaces the questionable assumption of diurnally constant cloud conditions made in the conventional DIEP by assuming a diurnal variation of cloudiness corresponding to the mean long-term diurnal variation of the planetary albedo. Validation of CDIEP, using the three-satellite Earth Radiation Budget Experiment (ERBE) data for December of 1986, indicates that on regional scales monthly time sampling errors for single satellite products are generally reduced but not completely removed in comparison with the currently applied diurnal model (EDIEP). On a global scale, rms errors are reduced by 16% and 28% for ERBE NOAA-10 and NOAA-9 monthly mean RSR, respectively. The efficiency of CDIEP is satisfactory by accounting for coherent diurnal variations of cloudiness, if present, and by reproducing the results obtained by EDIEP elsewhere.

Applying CDIEP to the full-year record of ScaRaB-Meteor ERB measurements enables the analysis of its impact with regard to the varying local observation time of each month. The standard deviation between regional monthly means of the RSR calculated by CDIEP and EDIEP varies between less than 2 W m−2 and about 4 W m−2 for high-noon and near-terminator time sampling conditions, respectively. On regional scales, time sampling errors with a 3½-month period, induced by the orbit’s precession, can be reduced, in particular for marine areas characterized by persistent stratocumulus, where the amplitude often exceeds 10 W m−2.

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Prince K. Xavier, Jean-Philippe Duvel, and Francisco J. Doblas-Reyes

Abstract

The intraseasonal variability (ISV) of the Asian summer monsoon represented in seven coupled general circulation models (CGCMs) as part of the Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) project is analyzed and evaluated against observations. The focus is on the spatial and seasonal variations of ISV of outgoing longwave radiation (OLR). The large-scale organization of convection, the propagation characteristics, and the air–sea coupling related to the monsoon ISV are also evaluated. A multivariate local mode analysis (LMA) reveals that most models produce less organized convection and ISV events of shorter duration than observed. Compared to the real atmosphere, these simulated patterns of perturbations are poorly reproducible from one event to the other. Most models simulate too weak sea surface temperature (SST) perturbations and systematic phase quadrature between OLR, surface winds, and SST—indicative of a slab-ocean-like response of the SST to surface flux perturbations. The relatively coarse vertical resolution of the different ocean GCMs (OGCMs) limits their ability to represent intraseasonal processes, such as diurnal warm layer formation, which are important for realistic simulation of the SST perturbations at intraseasonal time scales. Models with the same atmospheric GCM (AGCM) and different OGCMs tend to have similar biases of the simulated ISV, indicating the dominant role of atmospheric models in fixing the nature of the intraseasonal variability. It is, therefore, implied that improvements in the representation of ISV in coupled models have to fundamentally arise from fixing problems in the large-scale organization of convection in AGCMs.

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Prince K. Xavier, Jean-Philippe Duvel, Pascale Braconnot, and Francisco J. Doblas-Reyes

Abstract

The intraseasonal variability (ISV) is an intermittent phenomenon with variable perturbation patterns. To assess the robustness of the simulated ISV in climate models, it is thus interesting to consider the distribution of perturbation patterns rather than only one average pattern. To inspect this distribution, the authors first introduce a distance that measures the similarity between two patterns. The reproducibility (realism) of the simulated intraseasonal patterns is then defined as the distribution of distances between each pattern and the average simulated (observed) pattern. A good reproducibility is required to analyze the physical source of the simulated disturbances. The realism distribution is required to estimate the proportion of simulated events that have a perturbation pattern similar to observed patterns. The median value of this realism distribution is introduced as an ISV metric. The reproducibility and realism distributions are used to evaluate boreal summer ISV of precipitations over the Indian Ocean for 19 phase 3 of the Coupled Model Intercomparison Project (CMIP3) models. The 19 models are classified in increasing ISV metric order. In agreement with previous studies, the four best ISV metrics are obtained for models having a convective closure totally or partly based on the moisture convergence. Models with high metric values (poorly realistic) tend to give (i) poorly reproducible intraseasonal patterns, (ii) rainfall perturbations poorly organized at large scales, (iii) small day-to-day variability with overly red temporal spectra, and (iv) less accurate summer monsoon rainfall distribution. This confirms that the ISV is an important link in the seamless system that connects weather and climate.

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